A Photonic Crystal (PC) is a microstructured material with wavelength- and angle-dependent optical properties. For a Photonic Band Gap (PBG) to exist within a PC, the allowed quantum energy bands must not overlap for some area, and that gap area is the PBG. The PBG may exist in one, two, or three dimensions and in Transverse Electric and/or Transverse Magnetic modes. A complete 3-D PBG is a gap in the allowed quantum states across all propagation directions and polarization modes. The band gap position and gap width may be easily modified to yield desired photonic properties by varying crystal parameters, such as: structural geometry, crystal lattice dimensions, or contrast of indices of refraction between the composite material and vacuum/air. PBG properties are generally fully established within only a few lattice constants. Because of these properties, PBGs are attractive for thermally stimulated emitters for lighting and TPV applications and for sharp filters and lossless waveguides in both telecommunication and optical computing applications. An in-depth overview of PCs, PBGs, and fabrication methods can be found at: Cefe Lopez, “Materials Aspects of Photonic Crystals”, Advanced Materials 15, p 1679 (2003) or http://ab-initio.mit.edu/photons/tutorial/.
A complete 3-D PBG may exist in a variety of PC structures. One such structure is a stacked array of rods, known as a woodpile structure. Woodpile structures are fabricated using nanolithographic techniques modified from well-known semiconductor processes. These structures must be painstakingly assembled one layer at a time. For each layer, one must create a pattern, etch a mould, fill a mould, and polish that layer into a plane. In addition, the required feature size for visible emission applications is on the order of 100 nm; state-of-the-art lithography and very exceptional layer-to-layer registration quality control are required. This method is too costly in terms of time, capital, and material to be suitable for most applications. Further reference is available at U.S. Pat. No. 6,358,854 B1 “Method to fabricate layered material compositions”. Therefore, a system and method to fabricate low cost PBGs is required.
Another complete 3-D PBG structure is an inverse opal. An inverse opal is the volumetric inverse of an opal. An opal, which is a PC structure without a PBG, is a closely packed array of uniformly-sized spheres. An inverse opal is built by using an opal as a template and filling the residual (interstitial) volume between the spheres themselves with a material whose refractive index contrast to vacuum/air is high. For visible to near-IR light emission applications, the required lattice constants are in the range of 300 nm to 2 μm. Unfortunately, there are currently few suitable methods to economically fabricate materials within this size range. Molecular-based templates are too small, while traditional mechanical manipulations are too large, although they can be useful in microwave applications. Even though natural opal gemstones are of about the right particle diameter, they are impractical due to: too small of crystal domain size, particle size variations, limited availability, and extremely high cost.
Since there are limited templates available that are suitable for micromoulding, one must be synthesized. One method to synthesize an opal structure is via a controlled withdrawal process: taking a colloidal suspension of spheres (typically silica or polystyrene), inserting a substrate into the suspension in order to create a meniscus line, and then slowly evaporating the suspending agent (typically water). The surface tension of the evaporating water at the top of the meniscus line pulls the spheres into a closely packed array no more than a few layers thick, leaving an opal structure of spheres. Variations include angling the substrate and establishing a thermal gradient across the substrate to produce a higher quality sample. Generally, only a few layers are deposited; however, increasing the colloid concentration can result in deposits several layers thick. This is a slow process, taking days to months to grow a sample of commercially-useful size and quality. Faster withdrawal of the substrate results in faster growth rates but at the expense of more defects. Various use of walls, channels, and laser confinement have attempted to produce optical waveguides, couplers and other components. These have been plagued by high defect rates.
Another synthesis process includes centrifuging a suspension of spheres to produce an opal sediment. The centrifugal force packs the spheres into a closely packed array at the bottom of the centrifuge tube. As a result of typical tube geometries, the sediment is very thick and does not cover a large surface area. Domain sizes are small. Since PBG properties are fully developed in only a few lattice constants, thick structures not only cost more due to additional material but also offer no benefit since most applications desire a thin sediment over a relatively larger surface area.
Yet another opal synthesis process includes sedimentation using electrophoresis. A suspension of surface-charged spheres is placed between two plates, and as a voltage is applied between the two plates, the charged spheres are attracted toward the oppositely charged electrode, thereby modifying the sedimentation rate. The electric force may oppose the gravitational force, slowing the sedimentation rate. The slower sedimentation allows an opal to form. Fast sedimentation results in a random sludge. This process is further described in US2003/0,156,319 A1 “Photonic Bandgap Materials Based on Silicon”.
Yet another opal synthesis process applies a several kilohertz or greater AC electric field to a colloidal suspension of spheres via four hyperbolic electrodes. The AC field creates standing waves which tend to agglomerate spheres into thin layers with some degree of order. No means is provided to remove defects.
Naturally-occurring opals occur only via slow gravitational settling. Their rarity is a function of relatively uniformly sized naturally occurring silica spheres settling slowly enough to self-order into an opal structure.
Attempts at opal synthesis using a paramagnetic spheres in a magnetic field have not been successful. Only lines of spheres or two-dimensional pyramids containing a very small number of spheres have been produced. Another attempt has added non-magnetic spheres to a ferro-fluid, resulting in a sparse hexagonal structure, stable only under the applied field, not the Hexagonal Close Packed opal structure useful for PBGs.
In all of the above opal structure fabrication methods, high quality samples take a very long time to grow; lower quality samples can be grown faster. The largest samples are on the order of only a few square centimeters. Another limitation is that the opals are very fragile, easily damaged by handling. The opal structures must be dried before they are inverted. Larger area samples tend to crack more due to shrinkage when drying. The spheres may be slightly fused, creating a neck between the spheres, adding structural stability. Necking can have the added benefit of optimizing the width of the band gap. Silica opals are necked by drying the opal structure and then sintering, slightly melting the spheres such that they partly fuse together. Therefore, a system and method to fabricate opal structures quickly, with low defects, large domain size, and at low cost is needed.
An inverse opal structure can be formed from a synthetic opal structure in several ways. One such method starts with an opal structure made with silica spheres. Then, a Chemical Vapor Deposition (CVD) of Si, Ge, or other metal is applied in the interstitial spaces between the spheres, and the silica spheres are then etched out with a hydrofluoric acid solution. This results in a structure exhibiting a complete 3-D PBG. This process does not completely fill the interstitial spaces, creating only a thin layer of material or ‘eggshell’ around the spheres. Thicker shells are possible, but require notably longer deposition times and are further limited by infiltration of the deposition gas. These thin materials are fragile and are not very thermally conductive. Further disadvantages include the use of toxic CVD gasses and hydrofluoric acid as well as the need for vacuum pressures during processing.
Another method of inverting an opal structure is based on a synthesized polystyrene opal structure. An electroplating electrolyte is infiltrated into the interstitial spaces between the spheres; a CdSe alloy is electro-deposited from the bottom up; and, subsequently, the spheres are removed by dissolution in toluene. The result is a CdSe inverse opal PBG crystal. One disadvantage is the risk of damage to the opal structure when the electrolyte is infiltrated. Further reference is made in “Electrochemically grown photonic crystals”; Nature; Dec. 9, 1999. Therefore, a system and method to fabricate high quality, low-cost inverse opal structures and inverse opal PBG crystals is needed.
Thermal stimulation of a PBG material benefits applications including: lighting, ThermoPhotoVoltaic power generation, and thermal signature modification. Since PBGs modify the available quantum mechanical states within the structure, they do not behave as black bodies when thermally stimulated. Instead they produce a narrowband optical emission. The presence of a PBG modifies the available quantum states and thus the quantum statistical likelihood of emission of a particular wavelength. It is well known that the width of the PBG, and hence the thermal emission spectra, is a function of the structure geometry and of the complex dielectric constant contrast. The quantum mechanical form of Plank's law must be used, as the simplifications used to derive the classical black-body form do not apply. As with any thermal stimulation, the radiated energy is not coherent.
A Tungsten woodpile structure has been thermally stimulated in U.S. Pat. No. 6,583,350 “Thermophotovoltaic energy conversion using photonic bandgap selective emitters”. However, fabrication of a woodpile structure is too costly to be commercially accepted in a commodity market. An effective thermally-stimulated inverse opal structure has not yet been disclosed. An ‘eggshell’ CVD inverse opal structure suffers from a high thermal resistance and may lack a substrate, which is required in direct heating applications. The electro-deposited CdSe inverse opal structure will melt at high temperature and will evaporate in high vacuum.
The PBG is capable of significantly spectrally shaping thermally stimulated emissions. The use of a PBG as a replacement to a filament in a standard incandescent light bulb offers substantial energy savings. Common Tungsten filaments slightly shape the spectral output but have the general characteristic of a black body radiator including significant infra-red emissions. A PBG may have a luminous efficacy of 200 lumens per Watt, whereas a common Tungsten filament is about 17 lumens per Watt. This offers substantial advantage over common Tungsten filaments.
A woodpile PBG light source has been disclosed in “A three-dimensional photonic crystal operating at infrared wavelengths”, Nature 394, 251 (1998) and claimed in U.S. Pat. No. 6,768,256 “Photonic crystal light source”. It is well known that the position of the PBG is a function of lattice constant. While it appears simple to scale this structure down to visible wavelengths, unfortunately, this requires about 100 nm features and a modified state-of-the-art semiconductor foundry to fabricate. This results in a costly and a lengthy manufacturing process which is not suitable for a commodity market.
Therefore, a method for fabricating a low cost, high quality, high temperature, low vapor pressure PBG suitable for use in a commodity markets is needed.